1. Field of the Invention
This invention relates to prostheses for replacing or augmenting a nucleus pulposus of an intervertebral disk, and more particularly to apparatus for injecting a low modulus spinal implant into an intervertebral disc through a small portal.
2. Brief Description of the Prior Art
Chronic back pain, typically lower back pain, caused by injury or age-related degeneration of an intervertebral disc is a condition experienced by many patients.
Current treatment options for back pain range from conservative bed rest to highly invasive surgical procedures including spinal fusion and total disc replacement.
The human intervertebral disc is comprised of two major structures, an outer or peripheral tendinous structure, referred to as the annulus fibrosus or annulus, and an inner gelatinous nucleus pulposus located in a generally central region within the annulus fibrosus. Degeneration of the nucleus leads to disc degradation and loss of function. Consequently, another surgical option for the relief of back pain is replacement of the nucleus, leaving the annulus intact. The aim of nucleus replacement is to relieve pain, to restore healthy physiological function to the disc, and to prevent additional wear and degeneration of the annulus.
In view of the gelatinous nature of the nucleus pulposus, the use of hydrogels to replace the natural nucleus pulposus has been proposed, and materials and methods for such replacement have been proposed.
Hydrogels are typically formed from solid, generally insoluble hydrophilic polymers and, in their hydrated state, have a generally water-swollen structure. It has been proposed to design hydrogel implants that may have mechanical properties which approximate those of the natural nucleus pulposus, and to implant such hydrogel prostheses into the central region of an intervertebral disc, i.e., into the cavity normally occupied by the nucleus pulposus. Accordingly, a need has continued to exist for a method of determining the proper amount of a hydrogel prosthesis to be implanted in order to restore to the extent possible the natural mechanical properties and behavior of the intervertebral disc and for inserting such a hydrogel prosthesis.